The electric field or electric field strength is the electrostatic force acting on a small positive test charge placed at that point. If is the electrostatic force experienced by a test charge q at a point, then the electric field intensity at that point is given by

S.I unit of electric field intensity is Newton/coulomb (NC^{-1}).

If the test charge is not small, then the electric field may be affected by the test charge and hence we modify the above equation as follows:

Consider a system of charges *q*_{1}, q_{2}, ………..q_{n} placed at distances r_{1}, r_{2}….r_{n} with respect to some origin. Then the electric field intensity due to all these charges at a point is found out using the Principle of superposition. Let intensity due to the number of charges q_{1}, q_{2}, ………..q_{n}.
Then the resultant electric intensity at that point due to these charges is given by the superposition theorem.

Electric field intensity due to the n^{th} charge is

Magnitude of the electric field intensity is given by the equation:

Example1: Two point charges of 1*μ*C and -1* μ*C are separated by a distance of 100 Å. A point P is at a distance of 10 cm from the midpoint and on the perpendicular bisector of the line joining the two charges. The electric field at P will be

(a) 9 N/C

(b) 0.9 V/m

(c) 90 V/m

(d) 0.09 N/C

**Solution: **The point lies on equatorial line of a short dipole. So,** **

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**Example2:** Three charges 2q,-q and –q are located at the vertices of an equilateral triangle .At the center of the triangle

(a) the field is zero but potential is non-zero.

(b) the field is non-zero ,but potential is zero.

(c) both field and potential are zero.

(d) both field and potential are non zero.

**Solution: (b) **

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Obviously, E ≠ 0.Hence the field is non-zero but potential is zero.

**Example3:** ABC is an equilateral triangle. Charges *+ q *are placed at each corner. As shown in figure. The electric intensity at centre O will be

**Solution**: Unit Positive charge at O will be repelled equally by three charges at the three corners of triangle

By symmetry, resultant at O would be zero.